New 3D Interconnection Technology for Wearable Bioelectronics in the Future

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Guidance: Scientists have developed stretchable metal composites and printed them in 3D on soft substrates at room temperature. By implementing a thinner 3D interconnection, this research can not only enhance its technical functions, but also help to completely change the appearance of smart devices.

Development of Wearable Biotechnology Industry in the Future

Just throw a smart watch on your wrist to make you look cool, and the days are over. The wearable biotechnology industry has recently revealed its insatiable desire for future goods. Pain relief goggles, EEG monitoring, vital signs monitoring stickers, and even mind presbyopic glasses. They are just a small part of the latest project discussed at the 2019 Wearable Technology, Digital Health and Neurotech Silicon Valley Conference. It is uncertain whether all these wearable prototypes will catch on, but one thing is clear: there are more in the wearable technology arena. However, this huge potential is hampered by technological constraints: these wearable devices never really make users feel “wearable”.

Although they should feel like the second layer of the wearer’s skin, it is technically impossible to design “wearable” devices that are flexible and stretchable, and maintain good data recording capabilities on soft and curved skin. Wearable smart devices collect human biometric values by connecting electrodes to skin surfaces. Inside the device is a three-dimensional electrode wiring (i.e. interconnection) for transmission signals. So far, not only wiring can be formed on hard surfaces, but also parts of such wiring can be formed into fine and difficult-to-stretch metals, such as gold, copper and aluminium. In a paper published today in the Journal Nano Letters, a joint research group (UNIST) led by Professor Jang-Ung Park of the nanomedical center of the Daejeon Institute of Basic Sciences (IBS) of Korea and Professor Chang Chang Lee of the National Institute of Science and Technology of Ulsan, South Korea, is reported to have completely convertible electrode materials in Ulsan, South Korea. It has high conductivity. It is noteworthy that the new composite material is ultra-thin, with a diameter of 5 microns, which is half the width of traditional wire welding. By implementing a thinner 3D interconnection, this research can not only enhance its technical functions, but also help to completely change the appearance of smart devices.

Using Liquid Metal (LM) as the Main Substrate

The team used liquid metal (LM) as the main substrate because LMs are highly stretchable and have relatively high conductivity similar to solid metals. In order to improve the mechanical stability of metal liquids, carbon nanotubes (CNTs) are uniformly dispersed. “To make CNT uniformly disperse in liquid metals, we chose Pt because it has a strong affinity with CNT and LM because of the mixer and its working principle,” said Young-Geun Park, the first author of the study.

New Interconnection Technology

The research also shows a new interconnection technology, which can form a high conductivity 3D structure at room temperature: because of its high conductivity, the new system does not need any heating or compression process. In addition, the soft and stretchable properties of the new electrode make it easy to pass through the nozzle with fine diameter. The team used nozzles to print various 3D patterns directly. Park explains, “Forming high conductivity 3D interconnects at room temperature is an essential technology that can use a variety of flexible electronic materials. Lead bonding technology used in existing electronic devices utilizes heat, pressure or ultrasound that may damage software to form interconnected, skin-like devices.

Direct Printing Method

Using the direct printing method, the high-resolution 3D printing of this composite forms free-standing, wire-like interconnects. This new stretchable 3D electrical interconnections specifically consist of super-thin wires, as fine as 5 micrometers. Previous studies on stretchable metals have only been able to present wire lines of several hundred micrometers in diameter. The new system is even thinner than the interconnect of conventional wire bonding. Professor Jang-Ung Park, the corresponding author of the study noted, “We may soon be able to say goodbye to those bulky skin-based interfaces as this freely-transformable, super-thin 3D interconnection technology will come as a big breakthrough to the industry’s efforts to produce ever compact and slim gadgets.” Blurring the boundary between the human body and electric devices, this new technology will facilitate the production of more integrated and higher-performing semiconductor components for use in existing computers and smartphones, as well as for flexible and stretchable electronic devices.”